This invention relates to a hydraulic pressure control device for use in the braking system of a vehicle, and more particularly to a device for controlling hydraulic pressure to be supplied to brake units, on which the control device is mounted, in accordance with a load on the vehicle.
In order for braking forces acting on the front and rear wheels to be distributed close to an ideal condition, recent vehicles, such as automobiles, are equipped with a hydraulic pressure control valve in a hydraulic line which connects a master cylinder, i.e. a source for generating hydraulic braking pressure, and wheel cylinders of braking units, such as disk brakes or drum brakes, mounted on rear wheels.
The hydraulic pressure control valve includes a control piston which actuates to repeatedly open or close the hydraulic line, once the input hydraulic pressure from the master cylinder reaches a predetermined level (cut point pressure), so as to regulate the hydraulic pressure issuing from the hydraulic control valve to the wheel cylinders to a value lower than an increasing value of the hydraulic pressure input into the control valve from the master cylinder, thereby effecting a braking force acting on the rear wheels which is smaller than that on the front wheels, whereby the distribution of the braking force on the front and rear wheels is brought close to an ideal condition.
In a vehicle, such as a truck, in which a force loading the rear wheels varies by a substantial amount between an unloaded condition and a loaded condition of the vehicle, there is provided a load sensing type hydraulic pressure control device (LSPV) which adjusts a cut point pressure to higher values in accordance with increases of the load of the vehicle so as to establish correspondingly higher braking forces on the rear wheels.
The hydraulic pressure control device of a load sensing type functions to urge the control piston of the hydraulic control valve disposed in a hydraulic line connecting the master cylinder and wheel cylinder toward a direction in which the hydraulic line is opened by a force proportional to a given load, thus adjusting the cut point pressure to higher value depending upon a given increase in load. There are two types of hydraulic pressure control devices having a load sensing ability. One is an indirect hydraulic pressure control device which utilizes a relative displacement between the vehicle body and the rear wheels, the relative displacement varying in proportion to a change of the load. The indirect hydraulic pressure control device includes a coil spring disposed between the vehicle body and the rear wheel. A force of the spring resulting from a relative displacement between the vehicle body and the rear wheel is utilized to actuate a link mechanism, for example, so as to urge a control piston of the hydraulic pressure control valve toward a direction which will open the hydraulic line. The other is a direct hydraulic pressure control device which includes a suspension spring having one end thereof in abutment with the control piston of the hydraulic pressure control valve so as to urge the control piston of the hydraulic pressure valve toward a direction also opening the hydraulic line. In this regard, reference should be made, for example, to Japanese Patent Publication No. 57-28095 which corresponds to U.S. Pat. No. 4,062,597.
The prior art hydraulic pressure control devices as described above, however, involve several problems as noted below.
In the first mentioned indirect hydraulic pressure control device, it is necessary to adjust the force of the aforesaid coil spring disposed between the vehicle and the rear wheel for biasing the control piston since the device utilizes a relative displacement between the vehicle body and the rear wheel. It is also necessary to periodically re-adjust the biasing force since the vehicle body generally tends to lower this force with time. Further, problems have been experienced in that the hydraulic pressure control device is rather complicated, and has a low durability due to the use of a link mechanism or the like.
On the other hand, the direct hydraulic pressure control device also has a complex in construction since it requires a special mechanism for allowing only a predetermined portion of the load of the suspension spring to be exerted on the control piston of the hydraulic control valve. Furthermore, and with a suspension having a suspension spring with a strut passing therethrough, it is not possible to mount such a suspension spring onto a desired position due to the presence of the strut.